1. Field of the Invention
The present invention relates to a filter device, and a method of producing the filter device.
2. Description of Related Art
A conventional filter device includes filtering elements having different meshes, which are arranged in series in a fluid flowing direction. A coarse-mesh filtering element is arranged at an upstream side of the fluid flowing direction, and a fine-mesh filtering element is arranged at a downstream side of the fluid flowing direction. Large foreign objects are collected by the coarse-mesh filtering element, such that efficiency of collecting foreign objects is increased. Further, distribution of the collected foreign object in the filter device becomes proper. Therefore, pressure loss can be restricted from being increased, while the amount of the collected foreign object is increased.
JP-A-2003-214270 discloses a filter device having a single filtering element. A part of the filtering element is compressed so as to make the part to have fine mesh. That is, one kind of filtering element is used in the filter device, and a fine-mesh portion is formed in a part of the filtering element.
In this case, an upstream part of the filtering element in a fluid flowing direction is not compressed. A downstream part of the filtering element in the fluid flowing direction is compressed to a wall face of a casing accommodating the filtering element. Compression ratio is gradually increased toward a downstream end of the filtering element. Due to the compression, density of the filtering element becomes high, and mesh of the filtering element becomes fine. The density of the filtering element is gradually increased toward the downstream end of the filtering element. That is, the filtering element has a density gradient.
However, a cross-sectional area of the compressed portion of the filtering element is decreased. The cross-sectional area is an area of a cross-section perpendicular to the fluid flowing direction. The downstream end of the filtering element corresponding to a fluid outlet of the filter device has the maximum density and the minimum cross-sectional area. That is, a cross-sectional area of a fluid passage is decreased as the density is increased, in the filtering device having the density gradient.
When the cross-sectional area is decreased, filtering area for collecting foreign objects is decreased. When the amount of the collected foreign object is increased, pressure loss of the filtering element is increased, because the filtering element is clogged. Further, fluid pressure at an upstream side of the clogged part in the fluid flowing direction is increased, and a flowing speed of fluid is increased, when the cross-sectional area is decreased. Thus, a push-out phenomenon may be generated, in which foreign object collected in the clogged part flows out of the filtering element due to fluid force. That is, foreign objects may be discharged out of the filter device.